1. Field of the Invention
The present invention relates to an electrically conductive paste for a via-hole and a method of producing a monolithic ceramic substrate using the same, and particularly, relates to an electrically conductive paste for filling a via-hole formed in a green ceramic sheet and a method of producing a monolithic ceramic substrate comprising filling via-holes with the electrically conductive paste.
2. Description of the Related Art
A monolithic ceramic substrate has been known in which green ceramic sheets provided with electrodes and circuits are laminated and the electrodes and circuits formed on these sheets are electrically connected with via-holes.
The monolithic ceramic substrate is fabricated by the following steps. Via-holes are formed in green ceramic sheets with a drill or punch and filled with an electrically conductive paste containing an electrically conductive metal powder. Electrodes and circuits are formed on surfaces of the green ceramic sheets by a screen printing process or the like using an electrically conductive paste for forming the circuits. A plurality of green ceramic sheets are laminated, pressed, cut into a given substrate size, and baked for sintering.
In the conventional process, however, when the via-holes are filled with the electrically conductive paste, unsuccessful filling including insufficient filling or excessive filling will occur, cracks and elevations of the conductive metal (the sintered conductive metal powder) will form in the via-holes, or cracks will form in the ceramic. The cracks and elevations of the conductive metal and the cracks of the ceramic cause the failure of conductivity and structural defects, resulting in deteriorated reliability of the monolithic ceramic substrate.
It is an object of the present invention to provide an electrically conductive paste for a via-hole which barely forms cracks and elevations of the conductive metal and cracks of the ceramic during the baking step and which imparts high reliability to the resulting monolithic ceramic substrate, and to provide a method of producing a monolithic ceramic substrate using the same.
The preferred embodiment of the present invention provides an electrically conductive paste for a via-hole comprising an organic vehicle and an electrically conductive metal powder coated with a resin which is insoluble in a solvent contained in the organic vehicle.
In the above described electrically conductive paste for a via-hole, the electrically conductive metal powder is preferably copper powder and the insoluble resin is preferably a crystalline cellulose.
The preferred embodiment of the present invention further provides a method of producing a monolithic ceramic substrate comprising the steps of forming via-holes in a green ceramic sheet; filling the via-hole with an electrically conductive paste containing an organic vehicle and an electrically conductive metal powder coated with a resin which is insoluble in a solvent in the organic vehicle; forming a laminate by laminating a plurality of the green ceramic sheets filled with the electrically conductive paste; and baking the laminate.
The preferred embodiment of the present invention further provides a monolithic ceramic substrate, comprising a ceramic substrate; a via-hole provided in the ceramic substrate; and an electrically conductive paste provided in the via-hole, the electrically conductive paste comprising an organic vehicle and an electrically conductive metal powder coated with a resin which is insoluble in a solvent in the organic vehicle.
In the above described monolithic ceramic, the ceramic substrate may have a laminated structure.
The electrically conductive paste for via-holes in accordance with the present invention has improved filling characteristics into via-holes. Further, in the production of a monolithic ceramic substrate, the coated insoluble resin delays shrinkage of the conductive metal powder in the baking step, and hence it prevents cracks and elevations of the conductive metal and cracks of the ceramic. Since the insoluble resin is decomposed in the baking step, the resulting monolithic ceramic substrate can maintain excellent solder wettability and plating characteristics.
In the electrically conductive paste for via-holes in accordance with the present invention, the use of copper powder as the conductive metal powder causes the specific resistance to decrease and migration to be prevented. Since it is inexpensive, the resulting monolithic ceramic substrate is economical and has high reliability.
In the electrically conductive paste for via-holes in accordance with the present invention, use of crystalline cellulose as the insoluble resin can reduce deterioration of conductive resistance because it has excellent decomposition characteristics.
In accordance with the method of producing a monolithic ceramic substrate using the electrically conductive paste for via-holes of the present invention, cracks and elevations of the conductive metal and cracks of the ceramic barely form in the baking step. The resulting monolithic ceramic substrate does not substantially have structural defects in the via-holes and thus has satisfactory conductivity and high reliability. Since the insoluble resin in the electrically conductive paste is decomposed in the baking step, the resulting monolithic ceramic substrate can maintain satisfactory solder wettability and plating characteristics.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.